Processes for the catalytic oxidation of hydrocarbons are well known, for example, the oxidation of methane to produce syngas; the oxidation of ethylene to produce ethylene oxide; the oxidation of ethylene and acetic acid to produce vinyl acetate; the production of maleic anhydride from the oxidation of butene, butane or benzene; the production of phthalic anhydride by the oxidation of naphthalene or o-xylene; the ammoxidation of propane to acrylonitrile.
The catalytic oxidative dehydrogenation of hydrocarbons is a known process for the production of olefins. An example of such a process is described in EP-A-0 332 289. In this process, a hydrocarbon and an oxygen-containing gas are contacted with a catalyst, which is capable of supporting combustion beyond the fuel rich limit of flammability. The hydrocarbon is partially combusted, and the heat produced is used to drive the dehydrogenation of the hydrocarbon feed into olefins. Optionally, a hydrogen co-feed is also burned, and the heat produced by this combustion reaction is used to drive the dehydrogenation of the hydrocarbon.
Generally, in a catalytic oxidative dehydrogenation process, the reactants (hydrocarbon and an oxygen-containing gas) are passed over the catalyst directly to produce olefin product. Typically, the hydrocarbon is a saturated hydrocarbon such as a C2-C10 saturated hydrocarbon, such as ethane or a mixture of saturated hydrocarbons such as a mixture of C2-C10 hydrocarbons, naphtha or gas oil. The hydrocarbon may be gaseous or liquid at ambient temperature and pressure but is typically gaseous.
It is desirable in processes for the catalytic oxidation of hydrocarbons to have effective mixing of the reactants and/or uniform velocity profile of the reactant mixture prior to contact with the catalyst. However, although the reactants (hydrocarbon and oxygen-containing gas) may be pre-mixed prior to being introduced into the reactor, the velocity profile of the reactant mixture is often still nonuniform. Such a non-uniform velocity profile can lead to an unstable reaction. For example, high velocity gas mixtures over only part of the catalyst can lead to a decrease in selectivity. Where the oxidation reaction is one which is carried out close to its flammable limit, such as in the catalytic oxidative dehydrogenation of hydrocarbons, low velocity gas mixtures over only part of the catalyst can result in flash-backs.
Moreover, the heat generated by the oxidation reaction is generally not distributed evenly, reducing the efficiency of the oxidation process and resulting in potential loss of product.